Polyvinyl alcohol (hereafter, also referred to as PVA) has been widely used as a packaging material because it is excellent in transparency, oil resistance, chemical resistance, and gas barrier properties against oxygen or the like. PVA has been often used as a packaging material for food, pharmaceuticals, industrial chemicals, agrochemicals, and the like whose properties are greatly affected by deterioration due to oxidation.
In addition, PVA has been also used as a film or sheet for temporary protection of products and parts because of its high water solubility. PVA used as a film for temporary protection, such as surface protection during metalworking, protection during vulcanization of rubber parts, and surface protection of resin molded articles, can be easily removed by water, warm water, or hot water. This enables omission of a step of peeling and discarding the protective films.
However, the water solubility of PVA is problematically lowered when PVA is subjected to a heat history during the surface protection. For example, in metalworking, the protective film may be subjected to a heat history in the step of heat pressing or the like. In such a case, PVA may not be sufficiently removed due to the lowered water solubility.
To overcome the situation, Patent Literature 1 discloses a technique of improving the water solubility after the heat history by adding a hydrazine compound and a polybasic acid or an alkali metal salt thereof to a polyvinyl alcohol resin.
However, according to the technique of Patent Literature 1, the film disadvantageously exhibits poor processability upon packaging chemicals due to its low flexibility and has insufficient water solubility, especially resulting in a long dissolution time thereof.
In the case of a water-soluble film used for packaging agrochemicals, chemicals, or the like, PVA is modified with carboxyl groups or blended with a plasticizer for improvement of the properties as a water-soluble film. However, such a technique disadvantageously causes gradual discoloration of the film to light yellow due to a heat history upon film formation or contact with chemicals filled therein.
To overcome the situation, Patent Literature 2 discloses a water-soluble film prepared by adding a plasticizer and sulfite to a polyvinyl alcohol resin. Such a film suffers less coloring upon film formation or less coloring with time due to contact with chemicals.
However, addition of sulfite to a water-soluble film cannot shorten the dissolution time. In addition, sulfite may react with a weak acid to generate sulfur dioxide gas, which causes offensive odors from the water-soluble film.